1. Field of the Invention
The present invention relates to a surface modification method for an aluminum substrate (inclusive of an aluminum alloy substrate) having Nixe2x80x94P (nickel-phosphorus) plating applied thereto, which is used mainly for hard disks or the like as memory media for computers. More particularly, it relates to a method for modifying the surface so that the corrosion resistance and hardness of the substrate be improved to meet requirements for a low flying height of e.g. a GMR (Giant Magnetic Resistive) head.
2. Discussion of Background
Heretofore, an aluminum substrate having Nixe2x80x94P plating applied on its surface (hereinafter referred to as a Nixe2x80x94P plated aluminum substrate) has been widely employed as a magnetic disk substrate. However, along with recent high densification of HDD, a low flying height of a magnetic head and a low noise of a medium have been required, whereby flatness of the substrate surface and an improvement of a modified layer on the substrate surface by polishing, have been strongly demanded. In HDD drive, a conventional MR head has been used at a flying height within a range of from 400 to 600 angstrom (hereinafter represented by A). However, a GMR head has now been used at a flying height of not more than 300 A for high density recording. In magnetic recording, spacing between the head and the medium gives a substantial influence over the recording and reproducing characteristics. A change in the spacing by 10% or more is likely to bring about an error in recording and reproduction. Therefore, with HDD having a flying height of 300 A, it has been required to suppress the maximum projection height of the medium surface to be at most 50 A.
Further, in order to satisfy the requirements for high densification of HDD, a super finish substrate has been widely used wherein the average roughness Ra and the maximum projection height Rt of the substrate surface are from 10 to 5 A and from 100 to 50 A, respectively.
In order to increase the surface hardness, an aluminum substrate for a magnetic recording medium usually has Nixe2x80x94P plating coated in a thickness of about 10 xcexcm on its surface. In its production process, plating is carried out in a thickness of about 12 xcexcm by electroless plating, followed by polishing for 2 xcexcm to obtain a necessary surface precision. Then, a washing step by means of a surface active agent or supersonic cleaning is carried out to remove abrasive grains, etc., and all aluminum substrates are inspected and then shipped. Accordingly, if the performance during the inspection is maintained until the sputtering of a magnetic film, there will be no problem in the yield in the production of magnetic recording media.
The Nixe2x80x94P plating on the aluminum substrate is usually carried out by the following process. Namely, Nixe2x80x94P can not directly be plated on an aluminum substrate. Therefore, the aluminum substrate is processed into a predetermined shape and subjected to removal of strain, annealing, cleaning such as degreasing and then zincate treatment to form nuclei for electroless plating. This treatment is to deposit Zn on the aluminum substrate. Zn is deposited spottedly on the aluminum substrate, and Nixe2x80x94P will be precipitated thereon by an action of local cells. Accordingly, at the initial stage in growth, Nixe2x80x94P is not yet in the form of a film, and it forms a dense coating film for the first time when it has grown to a thickness of from 1 to 5 xcexcm. This indicates a possibility that at the initial stage in growth, the electroless plating liquid is taken into growth defective portions. The electroless plating liquid contains sodium hypophosphite as a reducing agent, and a salt containing sodium such as sodium citrate as a buffering agent, and accordingly, if such a liquid remains in the plated coating film, it is likely to cause corrosion.
Industrial products can not be 100% faultless, and corrosion inevitably occurs. Therefore, for such Nixe2x80x94P aluminum substrates, a useful period of time after the production has been prescribed. With a GMR low flying head, the maximum projection height is restricted to at most 50 A to secure the SN ratio of the medium and the running stability of the head, and a new measure has accordingly been required. The present invention is intended to satisfy such requirements.
When the surface of a material is subjected to mechanical processing, the surface of the material receives some action by the force effective during the processing, by the generated heat or by the action of e.g. a polishing liquid, and the surface layer of the finished side will have a layer modified by processing which is different in nature from the lower layer material and which extends to a certain depth from the outermost surface (Nikkei Gijutsu Tosho S59 xe2x80x9cSurface Finish Polishing Technologyxe2x80x9d).
In the case of a Nixe2x80x94P aluminum substrate, the thickness of such a layer modified by processing is about 1 xcexcm, as shown in FIG. 1. The layer modified by processing has micro cracks and is gelled by a reaction with the polishing liquid and very soft. FIG. 1 is characteristic curves showing the relation between the depth and the hardness of the surface as between a case where treatment with the functional water according to the method of the present invention was carried out and a case where no such treatment was carried out, to the Nixe2x80x94P aluminum substrate.
Due to retention of the plating liquid at defective portions in the coating film and the layer modified by polishing processing, a Nixe2x80x94P plated aluminum substrate will have projections formed by corrosion on the surface as the time passes after the production, by a washing treatment by means of a usual alkaline or acidic water. By the sputtering of a magnetic layer, the retained liquid is removed by heating under vacuum during the sputtering, and the sputtered magnetic film will cover the surface, whereby there will be no reaction with moisture, gas or the like in the air, whereby there will be no corrosion except for a peripheral portion which is hardly sputtered.
Therefore, substrate manufacturers have prescribed a limitation to the useful period of time of the substrates. However, in order to satisfy the requirements for the improvement of the SN ratio or for reduction of the flying height, the useful period of time is further shortened, and it has been required to take an industrial measure.
The present invention has been made under the above-mentioned circumstances, and it is an object of the present invention to provide a surface modification method for a Nixe2x80x94P plated aluminum substrate, which makes stabilized production of an aluminum substrate possible for a magnetic recording medium suitable for a low flying height GMR head by suppressing corrosion due to the layer modified by processing and due to the alkaline liquid remaining at defective portions of the Nixe2x80x94P plated aluminum substrate.
The present invention provides a surface modification method for an aluminum substrate, which comprises treating a Nixe2x80x94P plated aluminum substrate with a functional water having a plus or minus oxidation-reduction potential for a predetermined period of time in a washing step after a polishing step of the Nixe2x80x94P plated aluminum substrate.
Heretofore, a functional water such as ionized water or water having oxygen gas or hydrogen gas introduced therein, has been used for washing of semiconductors or washing of liquid crystal glasses. However, the majority is intended for washing to carry out e.g. removal of attached fine particles in an alkaline range where the oxidation-reduction potential (ORP) is minus, and it has been rare that a functional water is used for the purpose of surface modification.
The present invention is characterized by the use of a functional water having a plus or minus ORP.
It has been confirmed that by the use of such a functional water, it is possible to prolong the effective period of time which used to be a problem with conventional Nixe2x80x94P plated aluminum substrates, and it is possible to obtain an improved effect for the yield in production by an improvement of the layer modified by processing.